Magnetic resonance imaging (MRI) is a common and well known technique for imaging the internal structure of objects and for medical diagnosis. Conventional MRI requires that the object to be imaged be placed in a uniform (typically to within 40 ppm) and strong (typically in the range of 0.5 to 1.5 Tesla) magnetic field. Generating such magnetic fields is difficult and expensive.
Prepolarized MRI (PMRI) is a recent technique which uses a strong, nonuniform pulsed magnetic field in combination with a weaker, uniform magnetic field to perform imaging. The strong, pulsed field is known as the polarizing field and it is produced by a polarizing magnet. The weaker, uniform field is known as the readout field and is produced by a readout magnet. PMRI is also referred to as switched-field MRI and is related to field cycling nuclear magnetic resonance (NMR) relaxometry.
In PMRI, the polarizing field is switched on briefly (about 0.01 to 2 seconds) to polarize the nuclear spins inside the object to be imaged. Then, the polarizing field is rapidly reduced at a rate faster than the decay rate of the nuclear spin polarization. The nuclear spin polarization is then analyzed in the readout magnetic field. The polarizing field causes the nuclear spin polarization to be greater than it would be with only the readout field. Reference can be made to U.S. Pat. Nos. 5,629,624 to Carlson et al., 4,906,931 to Sepponen, and 5,057,776 to Macovski concerning PMRI.
The polarizing magnet in a PMRI device can be a solenoidal electromagnet. Solenoidal electromagnets can be designed with a particular geometry so as to produce a maximum amount of magnetic field with a minimum amount of electrical power. However, such minimum power solenoidal magnets have several problems. A minimum power solenoidal magnet will have a relatively large material cost due to the large volume of wire required. Also, The inductance/resistance (L/R) time constant will be relatively long, resulting in slower operation of the PMRI device or requiring a more expensive power supply.
A concern with electromagnets is that they heat up during operation. The heat generated during operation limits the length of time that the magnet can be operated.
Another concern with electromagnets is that they are relatively expensive. Magnets represent a significant portion of the cost of a magnetic resonance imaging system.